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A new tissue-friendly head implant for neuroscience studies with monkeys

A new tissue-friendly head implant for neuroscience studies with monkeys

In 2005, Professor Roger Lemon, from University College London, received an NC3Rs project grant to develop tissue-friendly, stable head implants with reduced risk of infection for neuroscience studies using non-human primates.

Research details

Principal Investigator: Roger Lemon, Professor of NeurophysiologyOrganisation: Institute of Neurology, University College LondonAward: £16,996, in 2005, over 36 monthsTitle: Development of a new tissue-friendly head implant for neuroscience studies with monkeys

Case study

Monkeys are used to understand the control and coordination of hand movements

An important approach for studying brain function is to make recordings of the tiny electrical signals that are generated by the single neurons that make up the brain’s functional networks. In the brain’s motor network, analyzing the spike activity of single neurons is essential to understanding the neural control of skilled hand and finger movements and their disruption by disorders such as stroke, cerebral palsy and spinal injury. For some studies, macaque monkeys are used since they provide a relevant model for investigating how the sensorimotor system controls the hands during the performance of skilled motor tasks similar to those carried out by humans.

The monkey’s head is restrained using a device implanted on to the skull

Stable single cell recordings from the brains of macaques performing behavioural tasks require head restraint during the period of recording. Conventionally, this is achieved by surgical implantation of an inert metal device onto the animal’s skull under general anaesthesia, and training of the monkey to accept the head restraint. Although these methods reliably restrain the monkey’s head, such implants are not tissue-friendly, and eventually pressure on the implant leads to inflammation and infection, bone necrosis and instability or breakage of the implant. This has a significant impact on the monkey’s welfare and can require additional surgery to move or replace the implant. In addition to the animal welfare concerns, a failed implant can interrupt studies and in the longer term can lead to the decision to euthanase the monkey, wasting animals and impacting on scientific progress.

A new head implant which minimises tissue damage and infections

With NC3Rs funding, Professor Roger Lemon, Institute of Neurology, University College London, has developed and tested a refined method of head restraint which minimises the adverse impact on the monkeys and gives appreciable scientific advantages over conventional devices. Two refinements have been introduced. First, the new method employs non-invasive magnetic resonance imaging (MRI) to generate a three-dimensional model of the monkey’s skull, allowing design of a custom-made implant tailored for individual monkeys. Second, it uses tissue-friendly material for the construction of the implants. Since the new method of head restraint is MRI-compatible, further non-invasive imaging can also be carried out to more accurately target brain recording and stimulation, improving the precision and amount of behavioural and electrophysiological data obtained from each macaque.

The study investigated the suitability of two tissue friendly materials, HAPEXTM (an artificial bone analogue) and TECAPEEKTM (a thermoplastic compound). The new implants were then assessed for their mechanical stability, the level of infection in skin margins and other tissues in contact with the implant, skin recession using serial photography, and the health of the implant/bone interface. The HAPEXTM implant became infected and had to be removed after approximately eight months of recording.

This was not a long enough period to recommend it for further use. Instead, TECAPEEKTM implants were tested in three animals. To date all have remained stable, secure and relatively free of infection for the duration of the experiments (up to three years in the longest case) thus avoiding the animal welfare concerns associated with a failed head implant. The TECAPEEKTM implants also had other advantages in that they were lower in weight and easier to implant than conventional stainless steel devices and promoted an improved interface with the bone and surrounding skin tissue.

More data can be obtained from monkeys with refined head implants

Five articles reporting the findings from studies using the new head restraint device have been published and MRI, photographic and electrophysiological data shared through the CARMEN project. The head restraint technique was presented at the 2008 Wellcome Trust-sponsored NC3Rs workshop on refinement of the use of chronic implants. It has also been adopted by researchers at Newcastle University.